International efforts to resolve the bloody crisis in Syria are mired in disarray and uncertainty in the face of violence spilling over into Turkey and the near certain failure to meet Tuesday's UN-imposed deadline for government troop withdrawals and a ceasefire.

Amid reports of new attacks and more than 100 dead, it was clear that the six-point plan of the UN-Arab League envoy to Syria, Kofi Annan, for breaking the deadlock had stalled because of the insistent demand by the president, Bashar al-Assad, for written guarantees that "armed groups" would first lay down their weapons, and their swift refusal to do so.

Turkey's deputy foreign minister, Naci Koru, said bluntly that the 6am morning deadline was now void.

Ankara also voiced concerns after Syrian forces fired across the border into a refugee camp near the town of Kilis in south-western Gaziantep province. It was the first such incident since Turkey began sheltering thousands of Syrians last summer and the latest to fuel international alarm about the escalating crisis.Washington expressed outrage, saying the Syrian government appeared to have little commitment to Annan's peace plan.

Mohammad Abdelqader, a refugee in the camp, told Reuters he had witnessed the shootings on Monday and that two Syrians had been killed and two more had been injured. Turkey is understood to have immediately protested.

Turkey's foreign minister, Ahmet Davutoglu, has briefed the UN secretary general, Ban Ki-moon, and Annan on the refugee issue, and Annan is to visit the border on Tuesday. Two prominent US senators, John McCain and Joe Lieberman, are also expected to visit the camps.

Analysts suggested that having apparently missed one deadline for a withdrawal and ceasefire, it will not be easy for Annan to set another given the profound lack of trust between the Syrian parties. Speculation is rife about a Turkish plan B, perhaps involving the setting-up of a border buffer zone.

According to the UN, 9,000 people have been killed in Syria over the past 13 months. Reports from local activists said 60 people had been killed across the country on Sunday. On Monday activists said a further 30 people, including 17 children, had been killed by security forces during the bombardment of the central town of Latmana. The Syrian Revolution General Commission later reported 30 dead in the shelling of Tel Rafat in the Aleppo area. The local co-ordination committees gave an overall death toll of 104 dead. None of these could be independently verified.

Lebanon's al-Jadeed television channel said one of its cameramen, Ali Shaaban, had been shot dead near the border between the two countries.

Syrian troops fired about 40 rounds across the border into northern Lebanon, killing Shaaban, Associated Press reported. "If you see the car, you would think it was in a war zone," Shaaban's colleague, Hussein Khreis, told the station.

The effective collapse of the Annan plan, sponsored by the Arab League as well as by the UN, means that western and Arab governments are likely to seek to go back to the UN security council, though there is no sign that Russia and China will reverse their opposition to censuring or sanctioning the Assad regime. In February both vetoed a security council resolution on Syria. China did call on both the Syrian government and the opposition to comply with the Annan plan "to alleviate the current tense situation and facilitate humanitarian assistance", and "promote a political solution to the conflict".

The appeal from Beijing came as Syria's foreign minister, Walid al-Muallem, flew to Moscow for emergency talks on the crisis with his Russian counterpart, Sergei Lavrov.

Syrian state media insisted on Monday that the Assad government stood by its commitment to the Annan plan and blamed "armed groups" for failing to provide written guarantees that they would stop fighting before regime forces withdrew from cities.

The foreign ministry said: "Annan has not offered written guarantees to the Syrian government that the armed groups agreed to stop violence, nor has he offered guarantees that Qatar, Saudi Arabia and Turkey will commit to stop funding and arming terrorist groups."

Syrian opposition spokesmen said that Assad's demand for such guarantees was a ploy designed to buy time.

Scepticism about Assad's readiness to implement Annan's plan was widespread from the start, but the western governments who dominate the Friends of Syria group insisted it was the "only game in town", given Russian support for Assad and the lack of appetite for outside intervention after Nato's controversial role in Libya last year.

The regime's response suggested it was seeking an outright rebel surrender rather than a truce as a prelude to "Syrian-led" negotiations about future political arrangements. Many Syrian opposition activists and foreign analysts had argued that Assad could not implement the plan because it would trigger massive demonstrations that he would be unable to contain.

Human Rights Watch, meanwhile, said in a report that Syrian security forces had summarily executed more than 100 civilians and wounded or captured opposition fighters during recent attacks on cities and towns. Many of the incidents took place in March this year.

In the cases documented by the organisation, at least 85 victims were Syrians who did not take part in fighting, including women and children. The report describes in detail several cases of mass executions of civilians, including the killing of at least 13 men in a mosque in Idlib on 11 March, the execution of at least 25 men during a search-and-arrest operation in Homs on 3 March, and the killing of at least 47 people, mainly women and children, in three neighbourhoods of Homs on 11 and 12 March.